Friction is an incredibly important force in everyday life.  It is the resistance force to movement between the physical contact of two objects.  It allows us to plant our feet on the ground and walk forward and it allows us to grip items in our hands and pick things up.  But there are many more complex examples of friction which require very specialized study.  We want to optimize the friction between the road and automobile tires.  We want the frictional force to be strong enough to firmly grip the road without wasting too much energy in the form of fuel.  The chemical composition and shape of tires are studied to optimize this and to also understand how water and ice affect this friction.  Another example is in fluid dynamics.  We want to study the frictional force between hydraulic fluid and the walls of pressurized hydraulic tubing to minimize the friction between them.  This increases the effectiveness of the hydraulic system.